Study On The Element Segregation Control Of Inconel 718 Superalloys By Laser Additive Manufacturing

Inconel 718?hereinafter referred to as IN718?is a Ni-Fe-Cr based superalloy,widely used under the complex stress operation condition up to the temperature of 650°C for long-term service.Due to its excellent high temperature mechanical properties,IN718 superalloy is diffusely used in the high temperature components in the field like aerospace.At present,the traditional alloy processing method severely restricts the production and application of IN718 alloy.Laser Cladding?LC?technology,as an advanced laser additive manufacturing method,is widely used in the processing and remanufacturing of IN718 superalloys due to its fast cooling rate,grain refinement and metallurgical interface bonding.However,in the rapid laser solidification process,IN718 alloy produces a large amount of element segregation and Laves phase in the interdendritic regions,resulting in the crack initiation and mechanical properites deterioration of the alloy.Therefore,based on the advantages of laser cladding technology and the characteristics of IN718 superalloy,this paper intends to use two types of laser sources,i.e.,Gaussian laser source and near-uniform laser source with flat top characteristic to fabricate the laser additive manufactured IN718 superalloy.The depositing characteristics,microstructure,Nb element segregation and Laves phase formation behavior,mechanical properties and solid-liquid interface solidification characteristics of the cladding layer under the two laser sources were comparatively studied.Meanwhile,the numerical simulation was also conducted to simulate the thermal history and the non-equilibrium solute redistribution coefficient of the two cladding layers.The element segregation mechanism and influencing factors of laser cladding IN718superalloy were further revealed by the comperative study of both the experimental and simulation results.More theoretical basis and experimental proof were provided for the control of the element segregation behavior in the laser additive manufacturing of IN718superalloy.The main conclusions of this paper are drawn as follows:?1?Near-uniform laser deposited IN718?ELD-IN718?and Gaussian laser deposited IN718?GLD-IN718?are both well formed with low pores formation,no obvious crack generation,and excellent metallurgical bonding.The ELD-IN718 superalloy produce a relatively flat bottom of the molten pool,coarse dendritic growth and wide primary dendrite spacing of about 15.712?m.While,GLD-IN718 produce a relatively steep bottom of the molten pool,fine dendritic growth and narrow primary dendrite spacing of about 9.05?m.?2?The degree of elemental segregation and Laves formation of the ELD-IN718 cladding layer is lower than that of GLD-IN718.The average Laves phase content of the former is about 5.6 vol.%;while degree of the segregation of Nb elements and Laves phase formation in the GLD-IN718 cladding layer are both severer,which accountd for about8.12 vol.%production of Laves phase.?3?The numerical simulation results show that the temperature and the vertical temperature gradient in the GLD-IN718 molten pool were both greater than that of ELD-IN718,whereas the lateral temperature gradient of it is lower than that of ELD-IN718.The typical solidification characteristic of GLD-IN718 contribute to a lower non-equilibrium distribution coefficient of the solute,i.e.,the non-equilibrium distribution coefficient k_v of GLD-IN718 laser cladding layer is about 0.54⁰.57,lower than that of 0.55-0.58 in the ELD-IN718 cladding layer.The result is consistent with the experimental results of higher element segregation and Laves phase formation in the GLD-IN718.?4?Tensile test results show that the tensile strength of ELD-IN718cladding layer is about 953 MPa,which accounted for about 23%increase in the comparation with that about 775.6 MPa tensile strength of GLD-IN718 cladding layer.While,the microhardness test results show that the average hardness of the GLD-IN718 cladding layer is about 275HV_0.3,which is higher than that about 250 HV_0.3 hardness of the ELD-IN718.The increase of the Laves phase content increaseed in the hardness of GLD-IN718.However,the Laves phase formation depleted most of the strengthening elements of the?matrix,which eventually decreased in the tensile strength of the alloy.